Numerical Simulation On Falling Film Fluid Flow, Heat And Mass Transfer Of Aqueous LiBr Solution

The concept of a thin liquid film falling downward an inclined or a vertical wall has been widely used in traditional industries and hi-tech fields, such as falling film evaporator, condenser, falling film absorber and falling film reactor, nuclear cooling and micro-electronic diveces cooling, because it can enhance heat and mass transfer rates without incurring much flow resistance and power consumption. The flow of falling liquid film exhibits abundant hydrodynamical behaviors because of the inherent instability. Due to its importance and universality of application, it is extremely necessary to investigate the hydrodynamic properties, heat and mass transfer of falling film and the influencing factors.Absorber is one important component in absorption system and its characteristics have significant effects on the overall absorption efficiency. In this dissertation, the numerical study on heat and mass transfer in film absorption under laminar flow was conducted; The surface wave dynamics of vertical falling films under monochromatic-frequency flowrate-forcing perturbations was computed by VOF method. And then, the influence factors of operation conditions and physical properties on surface wave dynamics were investigated. Finally, the numerical simulation has been carried out on steam absorption by wave falling film.A model of simultaneous heat and mass transfer process in a water-cooled vertical plate absorber under laminar flow was developed using lithium bromide/water as the working pairs. The practical convective boundary condition at the cooling water side is considered. The convective heat transfer coefficient is assumed constant, and the coolant temperature changes linearly along its flowing path. The model can predict temperature and concentration profiles across the film as well as along the downstream distance, heat and mass fluxes. In addition, the effect of cooling heat transfer coefficient on mass transfer rate was investigated. In particular, the effect of variable physical properties on the absorption process was considered and discussed.Numerical simulation has been made on the wave evolution and flow dynamics of falling films of aqueous LiBr solution along a vertical wall. Volume of Fluid (VOF) model is used to track the free surface and Continuum Surface Force (CSF) model is used for dynamic boundary conditions considering the effect of surface tension. The characteristics of wave dynamics, such as streamwise velocity profile and wall shear stress, were analyzed. A small amplitude forcing disturbance was introduced at the inlet of the flow boundary. The simulation results show that at low frequency the small disturbance may give rise to a solitary rolling wave with a large amplitude and some small amplitude capillary waves. At high frequency the wave amplitude becomes small and accompanied capillary waves disappear. The waveform is nearly like the sinusoidal shape. Furthermore, the circulation flow was observed at the peak of solitary wave. In most sections of the wave the velocity profile is described by the self-similar parabolic form. In some unique regions, such as the first capillary wave trough, the velocity is negative, indicating the reverse flow in these areas. This is the same situation as that proved by the distribution of wall shear stress. Based on the principle of boundary layer separation, the mechanism leading to the origination of backflow phenomena was explained. And the mechanism of wave enhancement was analyzed.Based on the wave model of falling liquid film, the influence of operation conditions and physical properties to the dynamics of the wave was examined. Operation conditions include disturbance frequency, Re and wall inclination angle. Physical properties include surface tension and viscosity. The simulation results indicated that as the increase of the disturbance frequency, the transition of solitary waves, interacting waves and sinusoidal waves took place. Furthermore, as the increase of Reynolds number, the wave amplitude, wave length and intensity of recirculation increased. As the increase of wall inclination angle, the component of gravity parallel to the wall increased, thus the wave grew faster. Peak height decreased as the increase of surface tension and wave numbers of capillary waves in front of solitary wave increased. As the increase of viscosity wave shape changed from the solitary shape to the sinusoidal one.A model of simultaneous heat and mass transfer process of steam absorption by wavy falling film was developed. The temperature and concentration profiles at different streamwise position were presented. At the same time, the heat and mass flux, local heat and mass transfer coefficient along the downstream distance were also presented. The mechanism of wave enhancement was analyzed.